1. Field of the Invention
The invention relates generally to a fluid-actuated impact member or ram such as a pile driver and more particularly to a hydraulic control circuit for operating a fluid-actuated impact member or ram for driving piles and other related operations.
2. Description of Related Art
An impact member is a heavy mass suspended at the end of a support mechanism. The impact member is guided for reciprocal movement in a downward working stroke and an upward return stroke. A control system for such an impact member is disclosed in U.S. Pat. No. 3,408,897 entitled "Fluid Power Hammer Having Accumulator Means to Drive the Hammer Through Its Working Stroke Independent of the System Pump". That patent discloses an impact tool hydraulic control circuit which is self-regulating and has built-in safety features to avoid damage to the mechanism or to the circuitry. However, the control circuit disclosed in that patent has certain inadequacies. For example, as shown in FIG. 6 of the drawings of that patent, a trip rod mechanically pushes a control valve spool rod 196 to the fall position at a preselected height of the impact member 11, and when impact member 11 is falling, exhausting fluid must move a sleeve 198 in order to escape to the tank. When sleeve 198 moves, it compresses a spring 199 which also acts on a control valve spool rod 196 to shift sleeve 198 to the drop position against the force of a return spring 218. As long as fluid is passing through sleeve 198, it holds control valve spool rod 196 in the shifted position. However when impact occurs, internal spring 199 returns sleeve 198 to its normal position, and return spring 218 returns control valve spool rod 196 and sleeve 198 to the lift position. When impact tool 11 is operating in the free fall mode, automatic pilot control valve F and control valve A always move together and, therefore, essentially function as a single valve. Because of this arrangement, the spring holds the spool of automatic pilot control valve F in position by mechanical means. Sleeve 198 compresses spring 199 which pushes against the spool of valve F.
The hydraulic pilot arrangement for shifting the position of the control valve spool and holding the control valve spool in the shifted position in the control circuit of the invention is a principal difference between the hydraulic control circuit of the invention and the prior art control circuit. Additionally, the replenishment check valve in the hydraulic control circuit of the invention increases the efficiency of the operation of the impact member and the reliability of the hydraulic control circuit.